Positive Displacement Pump Types

Positive displacement (PD) pumps transfer liquid by repeatedly trapping and moving a defined volume from suction to discharge. Theoretical flow is determined by pump displacement and operating speed. Actual delivered flow can be lower because of internal leakage, called slip, and can also be affected by inlet conditions and fluid properties. The pump develops the differential pressure required by the system at the imposed flow, within its design limits. Because a PD pump continues to displace liquid when its discharge is restricted, the system requires suitable overpressure protection. PD pumps are broadly divided into reciprocating and rotary types.

Reciprocating Pumps

Reciprocating PD pumps use linear motion of pistons, plungers, diaphragms, or bellows to draw liquid into and discharge liquid from pumping chambers. Their flow is inherently cyclic, so pulsation, inlet piping, acceleration pressure, valve behavior, and pressure protection are important application considerations. The four categories introduced here are air-operated, steam, power, and controlled-volume pumps.

• Refer to ANSI/HI 10.1-10.5 and ANSI/HI 10.6 additional information on air-operated pump
• Refer to HI 8.1-8.5 for additional information on direct acting (steam) pumps.
• Refer to HI 6.1-6.5 and HI 6.6 for additional information on reciprocating power pumps
• Refer to ANSI/HI 7.1-7.5, ANSI/HI 7.6, and ANSI/HI 7.8 for additional information on controlled volume metering pumps

Representative image	Reciprocating PD Pump Type overview
	Air Operated Air-operated pumps, commonly referred to as AODD pumps, are reciprocating PD pumps driven by compressed gas, usually air, supplied from an outside source. Depending on the design, the pumping member may be a diaphragm, bellows, piston, or plunger. Alternating motion draws liquid into the pumping chamber and discharges it through inlet and outlet valves.
	Steam Direct-acting steam PD pumps use steam applied to a power piston to produce the reciprocating motion that drives a liquid piston or plunger. These pumps may be arranged horizontally or vertically and are commonly identified by their piston or plunger construction and the number of pumping elements.
	Power Reciprocating power PD pumps use an external driver and a mechanical power end to convert rotary motion into the linear motion of one or more pistons or plungers. The pumping elements alternately draw liquid into and discharge liquid from the pumping chambers through inlet and outlet valves.
	Controlled Volume Controlled-volume metering pumps (CVMP) are PD reciprocating pumps used for accurate liquid dispensing or dosing. They may use plunger, piston, or diaphragm liquid ends, with flow adjusted by changing stroke length, stroking speed, or both.

Representative image	Reciprocating PD Pump Type overview
	Air Operated Air-operated pumps, commonly referred to as AODD pumps, are reciprocating PD pumps driven by compressed gas, usually air, supplied from an outside source. Depending on the design, the pumping member may be a diaphragm, bellows, piston, or plunger. Alternating motion draws liquid into the pumping chamber and discharges it through inlet and outlet valves.
	Steam Direct-acting steam PD pumps use steam applied to a power piston to produce the reciprocating motion that drives a liquid piston or plunger. These pumps may be arranged horizontally or vertically and are commonly identified by their piston or plunger construction and the number of pumping elements.
	Power Reciprocating power PD pumps use an external driver and a mechanical power end to convert rotary motion into the linear motion of one or more pistons or plungers. The pumping elements alternately draw liquid into and discharge liquid from the pumping chambers through inlet and outlet valves.
	Controlled Volume Controlled-volume metering pumps (CVMP) are PD reciprocating pumps used for accurate liquid dispensing or dosing. They may use plunger, piston, or diaphragm liquid ends, with flow adjusted by changing stroke length, stroking speed, or both.

Rotary Pumps

A rotary PD pump has a pumping chamber containing rotating elements, such as gears, screws, vanes, pistons, or lobes. The rotating elements act with close running clearances and do not use separate inlet and discharge valves as part of the pumping principle. The main rotary types introduced here are sliding vane, axial piston, flexible member, lobe, gear, circumferential piston, flexible member, and screw pumps.

Refer to ANSI/HI 3.1-3.5, ANSI/HI 3.6, ANSI/HI 4.1-4.6, and ANSI/HI 9.6.9 for additional information on rotary PD pumps.

Representative image	Rotary PD Pump Type overview
	Sliding Vane In a sliding vane pump, vanes move in and out of slots in a rotor to form pumping chambers with the casing. As the rotor turns, chamber volume increases at the inlet to draw in liquid and decreases at the outlet to discharge it, producing nearly constant flow with relatively low pulsation.
	Axial Piston Axial piston rotary pumps use multiple pistons arranged parallel to the axis of a rotating cylinder block. Two designs are common. In a swash plate pump, the block rotates on the drive shaft axis and the pistons bear against an angled stationary plate. In a bent-axis pump, the cylinder block itself is inclined to the drive shaft. In both, the tilt angle sets the displacement, and both are built in fixed- and variable-displacement versions.
	Flexible Member Flexible member rotary pumps use a deforming flexible member to trap, transport, and discharge liquid. The category includes peristaltic pumps, which progressively compress and release a flexible hose or tube, and flexible-vane pumps, which use a rotating elastomeric member to form pumping chambers within the casing.
	Lobe Lobe pumps carry liquid in chambers formed between the rotor lobe surfaces and the pumping chamber wall. Both rotors are driven and synchronized by external timing gears, so the lobes do not contact one another while the rotor surfaces provide continuous sealing at close running clearances which lowers slip. Common rotor forms include bi-wing, tri-lobe, and multi-lobe designs.
	Gear Gear pumps transfer liquid in chambers formed between intermeshing gear teeth and the pump body. Common designs include external gear pumps, with two meshing external gears, and internal gear pumps, which use an inner and outer gear arrangement.
	Circumferential Piston Circumferential piston pumps are a distinct rotary positive displacement pump type. Circumferential piston pumps use two arc-shaped rotor ‘pistons’ that rotate withing the pump casing. The rotors are driven by external timing gears and do not contact each other. The long sealing path between the rotor pistons and the casing walls allows it to operate with thin liquids and at higher pressures.
	Screw Screw pumps use one or more helical rotors to form and move pumping chambers axially from inlet to outlet. Designs include single-screw pumps, including progressing-cavity pumps, and multiple-screw pumps, which may be timed or untimed.

Representative image	Rotary PD Pump Type overview
	Sliding Vane In a sliding vane pump, vanes move in and out of slots in a rotor to form pumping chambers with the casing. As the rotor turns, chamber volume increases at the inlet to draw in liquid and decreases at the outlet to discharge it, producing nearly constant flow with relatively low pulsation.
	Axial Piston Axial piston rotary pumps use multiple pistons arranged parallel to the axis of a rotating cylinder block. Two designs are common. In a swash plate pump, the block rotates on the drive shaft axis and the pistons bear against an angled stationary plate. In a bent-axis pump, the cylinder block itself is inclined to the drive shaft. In both, the tilt angle sets the displacement, and both are built in fixed- and variable-displacement versions.
	Flexible Member Flexible member rotary pumps use a deforming flexible member to trap, transport, and discharge liquid. The category includes peristaltic pumps, which progressively compress and release a flexible hose or tube, and flexible-vane pumps, which use a rotating elastomeric member to form pumping chambers within the casing.
	Lobe Lobe pumps carry liquid in chambers formed between the rotor lobe surfaces and the pumping chamber wall. Both rotors are driven and synchronized by external timing gears, so the lobes do not contact one another while the rotor surfaces provide continuous sealing at close running clearances which lowers slip. Common rotor forms include bi-wing, tri-lobe, and multi-lobe designs.
	Gear Gear pumps transfer liquid in chambers formed between intermeshing gear teeth and the pump body. Common designs include external gear pumps, with two meshing external gears, and internal gear pumps, which use an inner and outer gear arrangement.
	Circumferential Piston Circumferential piston pumps are a distinct rotary positive displacement pump type. Circumferential piston pumps use two arc-shaped rotor ‘pistons’ that rotate withing the pump casing. The rotors are driven by external timing gears and do not contact each other. The long sealing path between the rotor pistons and the casing walls allows it to operate with thin liquids and at higher pressures.
	Screw Screw pumps use one or more helical rotors to form and move pumping chambers axially from inlet to outlet. Designs include single-screw pumps, including progressing-cavity pumps, and multiple-screw pumps, which may be timed or untimed.